The invention relates generally to photovoltaic (PV) power generation, and more particularly to techniques for controlling power ramp rates of single or multiple PV systems/inverters associated with single or multiple PV arrays.
Photovoltaic cells generate direct current (DC) power with the level of DC current being dependent on solar irradiation and the level of DC voltage dependent on temperature. When alternating current (AC) power is desired, an inverter is used to convert the DC energy into AC energy. Typical PV inverters employ two stages for power processing with the first stage configured for providing a constant DC voltage and the second stage configured for converting the constant DC voltage to AC current. Often, the first stage includes a boost converter, and the second stage includes a single-phase or three-phase inverter system.
Single and three phase photovoltaic inverters generally require a two-stage conversion power circuit to convert the varying DC voltage of a PV array to the fixed frequency constant amplitude AC voltage of the grid. Traditional PV inverters use a DC link as the intermediate energy storage step, which means that the converter first converts the variable PV array voltage to a constant DC voltage and subsequently converts the constant voltage into an AC current at the line frequency and unity power factor that can be injected into the grid.
PV systems/inverters can be designed to work with single or multiple arrays of PV cells. The power output of a PV system/inverter is greatly influenced by solar irradiation conditions associated with individual PV cells/arrays. Utilities often have other power resources, such as thermal power plants to balance their electrical loads, thus accommodating variability in PV power output during intermittent solar radiation conditions. Thermal power plants may include, for example, coal and gas fired stations. Power fluctuations of PV systems due to sudden cloud covering or others are usually dealt with by adjusting power output of these thermal power plants to provide relatively constant overall power matching demands. However, it is often difficult to change power output of thermal power plants quickly enough. Changing of power output may be also referred to as ramping. Thermal power generators desirably require a ramp rate that does not impose excessive thermal stresses, and that accommodate the natural lag times involved in heating and cooling the heat transfer components. As an example, coal-fired power stations may take over 12 hours to start from cold, and, even when hot, may take 2 to 3 hours to be ramped from 0-100% of their rated power. Ramping down of such thermal power generators may require even slower rates to minimize risk of damaging plant components. Solar conditions, on the other hand, may change drastically in a relatively shorter time span. It is, therefore, desirable to control power ramp rates of PV systems/inverters taking into consideration the maximum prescribed power ramp rates of such other power resources.
It is possible to limit power ramp rates of individual or multiple PV systems at any level up to a maximum power value that is dependent on the current solar conditions. This is achieved by curtailing a portion of the power output, so that the power ramp rate does not exceed a maximum desired ramp rate. However this limits the capture of solar energy and increases the effective cost of energy of the PV system(s). Further, although power ramp rate control has been achieved to some degree by such techniques on individual generators and wind farms, the problem has not been addressed at a photovoltaic system level.
There is, hence a need for a technique to control power ramp rates at a photovoltaic system/subsystem level within limits prescribed by transmission system operators, while allowing maximum capture of solar energy by individual PV arrays.